1. Technical Field
This invention relates generally to electronically controlled powershift transmissions for large agricultural vehicles, and more particularly to a method for determining the load being experienced by the vehicle from the boost pressure developed by a turbocharger associated with an engine of the vehicle, and controlling the engagement of a plurality of proportionally modulatable clutches in accordance with the load being experienced by the vehicle.
2. Discussion
Electronically controlled powershift transmissions are used in a wide variety of large agricultural and earth moving vehicles. These transmissions typically include a plurality of clutches which may be gradually modulated into engagement, as well as a plurality of "on-off" clutches which are not proportionally modulatable. Such transmissions also typically provide for a plurality of forward gear ratios as well as a plurality of reverse gear ratios. Shifting between any of the forward or reverse gear ratios, or between neutral and a forward or reverse gear ratio, typically involves engaging various combinations of the proportionally modulatable directional clutches and the on-off clutches to achieve the desired forward or reverse gear ratio. Frequently an electronic transmission control system is employed for generating the current drive signals for such proportionally modulatable clutches. Examples of such control systems are disclosed in U.S. Pat. Nos. 4,855,913 to Brekkestran et al. and 4,425,620 to Batcheller et al., both of which are hereby incorporated by reference.
Engines associated with large agricultural or earth moving vehicles also frequently employ a turbocharger. As is well understood, turbochargers operate by receiving exhaust gasses from an exhaust manifold of the engine and using the exhaust gasses to drive a turbine of an input power section. The input power section is coupled to an output power section which also includes a turbine adapted to draw ambient air into the turbocharger. The ambient air drawn in is directed into the intake manifold of the engine and thereafter directed into the individual cylinders. The input power section and output power section are coupled together so that they always rotate synchronously. As the engine rpm increases, the rate of flow of the generated exhaust gasses increases which causes the turbocharger to draw in a greater amount of ambient air, and therefore develop a greater "boost" pressure in the intake manifold. Since the boost pressure increases almost as quickly as the engine torque developed, the boost pressure at any given time itself represents a very good approximation of the torque being generated at the same instant by the engine. Since engine torque increases generally in accordance with the loading on the vehicle, the boost pressure generated by the turbocharger represents a very good approximation of the current engine torque-- and thus the load being experienced by the vehicle--at any given time.
Determining the load the vehicle is operating under before a shift is made is important during certain shifts where the proportionally modulatable clutches are modulated into engagement to accomplish the shift. This is because the load on the vehicle influences how quickly the shift should be executed. For example, if the vehicle is lightly loaded, a rapid shift (involving very rapidly modulating the pertinent proportionally modulatable directional clutch into complete engagement) would cause the vehicle to "lurch" significantly as the shift is completed. This condition is stressful not only on the internal components of the powershift transmission but also on the drive line components of the vehicle. Moreover, the lurch produced by a too-quickly executed shift can add to operator fatigue as the vehicle is operated over a prolonged period of time where significant shifting is required.
Conversely, if the vehicle is heavily loaded when a shift is commanded by the operator, the vehicle will almost instantly decelerate producing a significant, momentary "jolt" as an off-going proportionally modulated clutch disengages while an on-coming proportionally modulated clutch gradually is modulated into complete engagement. This condition is also hard on the internal components of the transmission as well as the drive line components of the vehicle. Additionally, the speed of the vehicle and/or engine torque may drop significantly during the time interval between the off-going clutch disengaging and the on-coming clutch fully engaging, thus causing the engine torque to drop below the peak point.
Therefore, it is highly desirable to tailor the rate of engagement of the proportionally modulatable clutch which is modulated into engagement during a shift from one gear ratio to the next depending upon the load which is being experienced by the vehicle, and thus the engine, at the instant just prior to the shift being executed. If the vehicle is operating under a no-load condition, preferably the proportionally modulatable clutch should be modulated into engagement at a slower rate of engagement to produce a "smooth" shift, and to prevent a lurch of the vehicle as the shift is executed. If the vehicle is heavily loaded, the modulatable clutch should be modulated into engagement much more quickly than during a no-load condition to avoid sudden deceleration of the vehicle as the shift is executed. Still further, it would be preferable to modulate the proportionally modulatable clutch into engagement at a rate somewhere between that which would be used for a no-load condition and that which would be used for a full-load condition, to thus tailor the rate at which the clutch is modulated into engagement in close accordance with the approximated load being experienced by the engine.
It is therefore a principal object of the invention to provide a method and apparatus for controlling the shifting of a powershift transmission in accordance with the load being experienced by the engine to thereby produce significantly smoother shifts between various gear ratios of the transmission.
It is another object of the present invention to provide a method and apparatus for monitoring the boost pressure developed by a turbocharger associated with an engine of an agricultural vehicle and to use the sensed turbo boost pressure to obtain an approximation of the load being experienced by the engine just prior to initiating the commanded shift, and to control the rate of engagement of a proportionally modulatable clutch needed to effect the commanded shift at a rate of engagement in accordance with the approximated engine load.
It is still another object of the present invention to provide a method and apparatus for controlling the engagement of a proportionally modulatable clutch of a powershift transmission of an agricultural vehicle such that when a proportionally modulatable directional clutch is involved in executing a shift, the directional clutch is caused to be engaged at a rate of engagement which is dependent upon the load being experienced by the vehicle just prior to the shift being initiated.